Many aircraft that use gas turbine engines for propulsion have commonly used pneumatic starters. Such aircraft have an accessory turbine air motor coupled to each propulsion engine through a gearbox with reduction gearing to crank the propulsion engine.
Compressed air, supplied by a load compressor that is part of an on-board auxiliary power unit (APU) or an external ground cart, supplies compressed air to the turbine air motor through a pneumatic starter supply system that requires numerous air ducts, seals and air valves that are bulky and heavy. Furthermore, such pneumatic starter supply systems are complex, and such complexity reduces the reliability of the aircraft and increases maintenance costs.
In recent years, electric starters have been considered for cranking gas turbine propulsion engines. Incorporating an electric start capability does not appreciable add to the cost, weight and complexity of the electrical system since the infrastructure exists and an electrical starting system can make use of the existing components and wiring.
Although a dedicated electric starter motor with a suitable overriding clutch and associated reduction gearing in the gearbox can be used as part of the electrical starting system, the most desirable approach is to use a single dynamoelectric machine that is alternatively operable as a generator or a starter motor to eliminate the need for separate machines, multiple mounting pads, additional reduction gearing in the gearbox, the overriding clutch and associated ducting and valves. Such an approach is commonly referred to as a “starter/generator” system, and such systems have been available in various forms for a number of years.
Typically, high-power electronic control equipment has been necessary to make such starter/generator systems operational in ordinary aeronautical applications. Since most aircraft architectures require alternating current for supplying on-board electrical components, such as fans, motors, pumps and electronics, an alternating current (AC) generator is generally used as a starter/generator. A high power motor controller must be used to convert the available electrical power for starting to a variable frequency AC power supplied to the starter/generator to bring the engine up to self-sustaining speed, after which the starter/generator is used in its conventional mode as a generator.
Additionally, since most on-board AC components require a power source with an AC frequency that is constant or within a range of frequencies and the AC power from the starter/generator is proportional to engine speed that may vary over a wide range, high power variable frequency (VF) to constant frequency (CF) conversion equipment is generally required. Such conversion equipment generally converts the VF AC power from the generator to direct current (DC) power and then converts the DC power to CF AC power.
The use of such high-power motor controllers and power conversion equipment increases cost, weight and complexity of the starter/generator system and it reduces reliability. Thus, an alternative approach, as described in Kandil et al., U.S. Ser. No. 10/154,942, filed May 24, 2002, commonly owned by the assignee of this application and incorporated by reference, eliminates the use of high power motor control and power conversion equipment as part of the starter/generator by using a unique mechanical coupling system between the starter/generator and the engine that comprises a torque converter coupling the starter/generator to the engine for starting the engine and a constant speed transmission or drive coupling the engine to the starter/generator for generating power once the engine has reached self-sustaining speed.
The system as described in Kandil et al. is quite satisfactory for aircraft architectures that have all on-board electrical components operating at CF AC. However, some new architectures have on-board electrical components that require an adjustable range of frequencies, such as environmental control system (ECS) motors. The frequency of AC power for such components is adjusted according to flight conditions and requirements. For instance, the power frequency for such ECS motors changes to vary the speed of the motors to suit flight conditions as required by the ECS.
The use of the Kandil et al. starter/generator system in aircraft architectures that require adjustable VF AC power requires conversion of CF AC power to adjustable VF (AVF) AC power. Since electrical equipment, such as ECS motors, that require such AVF AC power can have significant power requirements, high power CF to AVF conversion equipment is necessary for their operation. This increases cost, weight and complexity of the system.
A co-pending application by Hoppe et al., owned by the assignee of this application and incorporated by reference, describes a starter/generator system for a gas turbine engine used in aeronautical applications that couples a single dynamoelectric machine to the gas turbine engine through a torque converter in a starting mode, and then disengages the torque converter and engages the engine to the dynamoelectric machine through an adjustable speed transmission in a generating mode after the engine reaches self-sustaining sustaining speed, wherein the speed of the adjustable speed transmission is set to match the frequency of AC generated by the dynamoelectric machine with on-board electrical equipment requirements to suit flight conditions. This system is very satisfactory for such aircraft architectures that require AVF AC power. However, in aircraft architectures that can tolerate a non-adjustable narrow range variable frequency (NRVF) AC power system, this approach is also overly complex, heavy and costly.
A co-pending application by Thomson et al., owned by the assignee of this application and incorporated by reference, describes a starter/generator system for a gas turbine engine used in aeronautical applications that couples a single dynamoelectric machine to the gas turbine engine through a torque converter in a starting mode and engages the engine to the dynamoelectric machine through a mechanical differential in a generating mode after the engine reaches self-sustaining speed and combines the output of the engine and the torque converter in the differential to regulate the frequency of AC generated by the dynamoelectric machine within a range of frequencies suitable for on-board electrical equipment by dynamically regulating the flow of hydraulic fluid to the torque converter.
Although the various architectures for starter/generator electrical systems as described above each have their advantages, selection of a single architecture is sometimes less than ideal. For instance, selection of a CF AC power system may seem necessary if a substantial number of on-board electrical components require CF AC. However, if AVF AC is required for other on-board components, such as the ECS motors, then high power CF to AVF conversion equipment is necessary for their operation. This increases cost, weight and complexity of the system.
Likewise, selection of a NRVF AC starter/generator system may seem wise when a substantial number of on-board electrical components tolerate a band of AC frequency power. The use of a NRVF AC starter/generator system architecture as described in Thomson et al. can greatly reduce cost, weight and complexity of the system. However, if some of the on-board components require AVF AC power, then the necessary high power NRVF to AVF conversion equipment offsets any advantage in reduced weight, cost and complexity of the NRVF system.